Data going through a channel in a serial communication link is subjected to distortion associated with the interference between symbols. As a bit that is transmitted as a pulse makes its way to a receiver through the communication channel, the pulse spreads itself out by the time it reaches the receiver and distorts its adjacent bits. This phenomenon of distortion is referred to as intersymbol interference, and commonly known by the acronym of ISI.
There are two components of intersymbol interference: post-cursor intersymbol interference and pre-cursor intersymbol interference. The distortion due to previous bits on the subsequent bits is referred to as post-cursor intersymbol interference and the distortion on the previous bits from the subsequent bits is referred to as pre-cursor intersymbol interference.